Iron regulation in Diphtheria toxin
Overview Diphtheria is an infection caused by Corynebacterium diphtheria bacteria. The disease presents as an upper respiratory tract illness with sore throat, low grade fever and an adherent membrane of the tonsils. It can quickly develop into lesions that cause respiratory distress and degenerative changes in heart, muscle, peripheral nerves, adrenals, kidneys, and liver. Pathogenicity The pathogenicity of Corynebacterium diphtheriae is caused by two phenomena: 1. Invasion of the local tissues of the throat after colonization and subsequent bacterial proliferation. 2. Toxigenesis: bacterial production of diphtheria toxin. The toxin causes the death of eukaryotic cells by ADP ribosylating translation elongation factor EF-2, blocking translation and ultimately releasing free iron. Toxigenicity: Iron regulation of tox gene The virulence factor of C. diphtheriae is the diphtheria toxin DT, its encoded by the tox gene. Interestingly the tox gene is not part of the bacterial chromosome; it is located on a bacteriophage that is present in the bacteria. Expression of the tox is regulated by a bacterial repressor, DtxR. Production of DT by C. diphtheria ''is modulated by extracellular iron concentrations. The tox gene is upregulated when C. ''diphtheriae is under conditions of iron deficiency. Boyd et al. found that DtxR gene repressed expression of the tox promoter under high iron conditions but failed to repress tox expression in low iron medium. This suggested that the prodct of the DtxR gene was an iron dependent repressor that negatively regulated the tox operon at the level of transcription. (2). Schmitt and Holmes further characterized the repressor gene for C. diphtheriae tox operon. They cloned the DtxR gene into the plasmid pCM2.6, the hybrid plasmids were transformed by electroporation into wild type C. diphtheriae and a mutant that produced the toxin under high iron conditions. They found that expression of cloned DtxR restored repression of DT production during growth in high iron medium. DtxR was characterized as a 29 kDa protein measured by SDS-polyacyclamide gel. (3) Conclusion The tox gene is regulated by a repressor product of the DtxR gene that in turn is activated by iron. The active repressor binds to the tox gene operator and prevents transcription. This is important for C. diphtheriae to infect cells, eukaryotic cells have no available iron, in that environment the bacteria can synthesize the toxin required for pathogenesis. In artificial culture the most important factor controlling the yield of the toxin is the concentration of inorganic iron present in the culture medium. This has practical importance for the industrial production of toxin to make the diphtheria vaccine. Under the appropriate conditions of iron starvation, C. diphtheriae will synthesize diphetheria toxin as 5% of its total protein. References 1. http://www.cdc.gov/diphtheria/index.html 2. Boyd J et al. Molecular cloning and DNA sequence analysis of a diphtheria tox iron dependent regulatory element (dtxR) from Corynebacterium diphtheriae. Proc. Natl. Acad. Sci. USA 1990. 87:5968-5972 3. Schmitt M and Holmes R. Iron dependant regulation of diphteria toxin and siderophore expression by the cloned Corynebacterium diphtheriae repressor gene DtxR in C. diphtheria C7 strains. Inf. Imm. 1991. 59:6. 1899-1904